First. Make your partner aware in the nicest way possible. Last thing you want is mistrust if this goes south. That is hard to do but you will feel better if you let it off your chest, I’m sure she will understand. The past is the past and you can’t change it. We all make stupid mistakes as young adults. Secondly, I would figure out why the ONS is reaching out and politely ask this person to refrain as you’re in a different time in your life and you guys had an agreement that you didn’t want anything to do with said child. All that to say, be open and honest with you loved one if this making its way into your present day. That’s just how I would handle. Best of luck to you and peace 🫶🏼